Internal combustion engines introduce an air/fuel mixture into each cylinder that is compressed during a compression stroke and ignited by either the compression (compression-ignition) or by a spark plug (spark-ignition). Spark-ignition (SI) engines may operate in different combustion modes, including, by way of non-limiting examples, a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines may also be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion.
An engine in-cylinder air/fuel mixture may be characterized in terms of an air/fuel ratio, which may be described as stoichiometric, lean, or rich. Compression-ignition engines and spark-ignition engines may be configured to operate at lean air/fuel ratio conditions. Engine exhaust gases associated with lean air/fuel ratio operation may have relatively higher quantities of nitrogen oxides, including, e.g., nitric oxide (NO) and nitrogen dioxide (NO2), as compared to an internal combustion engine operating at stoichiometry.